[PATCH] fpu: xstate: Keep xfd_state always in-sync with IA32_XFD MSR

From: Adamos Ttofari
Date: Thu May 11 2023 - 11:29:37 EST

Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and
commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a
per_cpu variable xfd_state to keep the IA32_XFD MSR value cached. In
order to avoid unnecessary writes to the MSR.

xfd_state might not be always synced with the MSR. Eventually affecting
MSR writes. xfd_state is initialized with 0, meanwhile the MSR is
initialized with the XFEATURE_MASK_USER_DYNAMIC to make XFD fire. Then
later on reschedule to a different CPU, when a process that uses extended
xfeatures and handled the #NM (by allocating the additional space in task's
fpstate for extended xfeatures) it will skip the MSR update in
restore_fpregs_from_fpstate because the value might match to already cached
xfd_state (meanwhile it is not the same with the MSR). Eventually calling a
XRSTOR to set the new state (that caries extended xfeatures) and fire a #NM
from kernel context. The XFD is expected to fire from user-space context,
but not in this case and the kernel crashes.

To address the issue mentioned initialize xfd_state with the current MSR
value and update the XFD MSR always with xfd_update_state to avoid
un-sync cases.

Fixes: 672365477ae8 ("x86/fpu: Update XFD state where required")

Signed-off-by: Adamos Ttofari <attofari@xxxxxxxxx>
arch/x86/kernel/fpu/xstate.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 0bab497c9436..36ed27ac0ecd 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -179,8 +179,14 @@ void fpu__init_cpu_xstate(void)
* key as that does not work on the boot CPU. This also ensures
* that any stale state is wiped out from XFD.
- if (cpu_feature_enabled(X86_FEATURE_XFD))
- wrmsrl(MSR_IA32_XFD, init_fpstate.xfd);
+ if (cpu_feature_enabled(X86_FEATURE_XFD)) {
+ u64 xfd;
+ rdmsrl(MSR_IA32_XFD, xfd);
+ __this_cpu_write(xfd_state, xfd);
+ xfd_update_state(&init_fpstate);
+ }

* XCR_XFEATURE_ENABLED_MASK (aka. XCR0) sets user features
@@ -915,7 +921,7 @@ void fpu__resume_cpu(void)

if (fpu_state_size_dynamic())
- wrmsrl(MSR_IA32_XFD, current->thread.fpu.fpstate->xfd);
+ xfd_update_state(&init_fpstate);


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